Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the progressive decline of memory and cognitive functions. Dementia associated with AD is referred to as senile dementia of the Alzheimer's type (SDAT) in usage with Alzheimer's disease. AD is characterized clinically by progressive loss of memory, cognition, reasoning, judgment, and emotional stability that gradually leads to profound mental deterioration and ultimately, death. Although there are many hypotheses for the possible mechanisms of AD, one central theory is that the excessive formation and accumulation of toxic beta-amyloid (Aβ) peptides either directly or indirectly affects a variety of cellular events and leads to neuronal damage and cell death (Selkoe, Neuron. 1991; 6(4):487-98 1991; Selkoe, J Clin Invest. 2002; 110(10):1375-81).
AD is a progressive disorder with a mean duration of around 8-15 years between onset of clinical symptoms and death. AD is believed to represent the seventh most common medical cause of death and affects about 5 million people in the United States. The prevalence is expected to reach 7.7 million by 2030. About 1 in 8 people over the age of 65, 13% of this population, have AD (Alzheimer's Association 2008 Alzheimer's Disease Facts and Figures). AD currently affects about 15 million people world-wide (including all races and ethnic groups) and owing to the relative increase of elderly people in the population its prevalence is likely to increase over the next two to three decades. AD is at present incurable.
To date, there is limited opportunity for prophylactic intervention for AD because of insufficient diagnostic methods. At present, a definitive diagnosis of AD requires observing lesions in the brain tissue of patients post-mortem or, rarely, in small biopsied samples of brain tissue taken during an invasive neurosurgical procedure. Nevertheless, physicians routinely attempt to distinguish AD from other forms of dementia based on a battery of symptoms, relying on the known correlation between such symptoms and the lesions observed in biopsies. Tests currently used to diagnose AD include combinations of qualitative questionnaires such as the Mini-Mental State Examination (MMSE), the Mini-Cognitive Examination, and the AD Cooperative Study-Activities of Daily Living Scale (ADCS-ADL); physical and neurological evaluation; and structural (MRI, CT) and functional brain imaging (PET; FDG-PET). These tests are typically conducted to rule out other disease or conditions rather than to provide a definitive diagnosis of AD.
Some methods exist for detecting pathogenic biomarkers for AD, such as Aβ, Tau, and Neural thread protein/AD7C in living subjects. For example, detection of Aβ in a living subject include direct (imaging) or indirect (biochemical) detection. In vivo imaging of Aβ can be achieved using radioiodinated flavone derivatives as imaging agents (Ono et al., J Med Chem. 2005; 48(23):7253-60) and with amyloid binding dyes such as putrescein conjugated to a 40-residue radioiodinated A peptide (yielding 125I-PUT-A 1-40). This agent was shown to cross the blood-brain barrier and bind to Aβ plaques (Wengenack et al., Nature Biotechnology. 2000; 18(8): 868-72). Imaging of Aβ also was shown using stilbene [11C]SB-13 and the benzothiazole [11C]6-OH-BTA-1 (also known as [11C]PIB) (Nicholaas et al., Am J Geriatr Psychiatry. 2004; 12:584-595).
Quantitation of Aβ (1-40) in the peripheral blood also has been demonstrated using high-performance liquid chromatography coupled with tandem mass spectrometry in a linear ion trap (Du et al., J Biomol Tech. 2005; 16(4):356-63). Detection of single Aβ protein aggregates in the cerebrospinal fluid of Alzheimer's patients by fluorescence correlation spectroscopy also has been described (Pitschke et al., Nature Medicine. 1998; 4: 832-834). U.S. Pat. No. 5,593,846 describes a method for detecting soluble Aβ. Indirect detection of Aβ peptide and receptor for advanced glycation end products (RAGE) using antibodies also has been described. Lastly, biochemical detection of increased BACE-1 activity in cerebrospinal fluid using chromogenic substrates also has been postulated as diagnostic or prognostic indicator of AD (Verheijen et al., Clin Chem. 2006; 52:1168-1174). Other methods include detection of Tau, and Neural thread protein/AD7C in the cerebrospinal fluid.
In an attempt to improve treatment and diagnosis of AD, numerous gene-expression profiles have been generated to compare genes expressed in post-mortem brain tissue with those expressed normal brain tissue using various techniques including microarray laser capture microdissection (Loring et al., DNA and Cell Biology. 2001; 20(11): 683-95; Mufson et al., Neurochem. Res. 2003; 27(10): 1035-48; Dunckley et al., Neurobiol Aging. Oct. 1, 2005; Brooks et al., Brain Res. 2007; 1127(1):127-35; Liang et al., Physiological Genomics. 2008; 33:240-256; Liang et al., Proc Natl Acad Sci USA. Mar. 10, 2008); Some gene-expression profiles using peripheral tissues, such as lymphocytes or fibroblasts, also have been generated in an attempt to identify gene expression profiles associated with familial (inherited) AD or evaluate the effect of a potential treatment on differentially expressed genes (Nagasaka et al., Proc. Natl. Acad. Sci. USA. 2005; 102(41): 14854-14859).
Current diagnostic measures for AD include identification of a clinical core of early, progressive and significant episodic memory loss plus one or more abnormal biomarkers (biological indicators) characteristic of AD, including atrophy (wasting) of the temporal lobe as shown on MRI; abnormal Aβ protein concentrations in the cerebrospinal fluid; a specific pattern showing reduced glucose metabolism on PET scans of the brain; and a genetic mutation associated with within the immediate family.
Like the physical and mental examinations, the foregoing methods are not yet totally reliable or accurate for a diagnosis of Alzheimer's because the same gene patterns are found in other diseases or conditions. As a result, the costs of diagnosing AD are enormous because of the numerous tests and specialists involved and because the inability to diagnose Alzheimer's in early stages precludes patients and their families from adequately planning for the future, increasing costs for long-term care. In addition, estimates rates of misdiagnoses or no definitive diagnosis are in the range of 50-75%.
There remains a need for a simpler way to achieve more definitive diagnoses of AD that are less expensive and invasive, more accurate, and which can be used at an earlier stage for quicker intervention. Importantly, because the neurodegenerative process and substantial cell loss likely begins well before manifestation of the cognitive symptoms of AD, an effective diagnostic test that could more accurately diagnose AD, including early AD and even a pre-disposition to AD, would be invaluable.